This invention provides a system for monitoring the condition of a device such as a pneumatic tire mounted on a rotating axis and providing indications of both satisfactory and unsatisfactory conditions at the operator's control station. In a pneumatic tire/vehicle system, it will respond to both excessive high tire pressure and dangerously low pressure to provide a warning signal to the vehicle operator, and will respond to a range of satisfactory pressures by providing an indication of satisfactory condition to the vehicle operator.
The value of maintaining proper inflation pressure in vehicle pneumatic tires has been demonstrated both in vehicle safety and in minimizing tire wear. Warning systems in the prior art have frequently addressed themselves to providing a signal only in case of low tire pressure and do not respond to excessive high pressure to indicate an unsatisfactory signal. Such systems are shown in Yamasaki--U.S. Pat. No. 3,828,309 and Kaida--U.S. Pat. No. 3,832,681. Excessive high pressure is also dangerous, particularly when one tire of a pair has excessive pressure, and it also leads to uneven tread wear and economic loss. In previous systems for indicating both high and low pressures, such as Ainsworth--U.S. Pat. No. 2,629,086, double air pressure chambers have been used leading to great complexity and fabrication cost.
Prior art tire pressure systems usually have wires connecting the pressure sensor to an indicator at the operator's station. Wiring is expensive to install and subject to failure in the hostile environment of a moving vehicle. Attempts to provide wireless transmitters of tire pressure signals have not been completely successful. Such transmitters require batteries which are subject to failure or they must be wired to a power source.
Koonce--U.S. Pat. No. 3,588,815 and Mills--U.S. Pat. No. 3,178,686 are examples of prior art which use expendable batteries mounted on the rotating wheels. The batteries not only introduce space, weight and location limitations but also create a maintenance problem in checking or replacing the batteries at regular intervals. Additionally, the need for indicating a satisfactory pressure at the beginning of each trip or on operator command in order to be assured of proper operation of the monitoring system will lead to rapid depletion of the batteries. Another method of supplying power in prior art, Ross--U.S. Pat. No. 3,786,413, has been the use of mechanical devices dependent on dynamic forces to generate electric current on each wheel for transmission of RF signals thereform. Still another prior art, Streglein--U.S. Pat. No. 3,694,803, has depended on the use of a passive transducer element powered by a space-transmitted radio frequency. Most prior art has required the attachment of an air hose or tube either directly to the pressurized cavity by means of a hole through the rim or to the valve stem of the wheel and thereby to the pressurized cavity. Experience has shown that the potential for air leakage is increased directly with the number of openings into the air cavity or valve stem. Therefore, a monitoring system which will not require the addition of openings will be most reliable with respect to possible loss of air pressure.
A typical pneumatic tire is designed to be inflated to a given pressure when cold and to operate satisfactorily at a higher pressure when the tire and the air it contains are heated by normal road operation. Hence, a range of satisfactory pressures must be accommodated in order for a pressure monitoring system to have maximum flexibility. When the pressure rises above or falls below that range, it should immediately provide an indication of malfunction and the type of malfunction (high or low) to the vehicle operator.
Many situations such as the checkout of an aircraft before flight require a positive signal that the tire condition is normal as well as a warning signal that the condition is abnormal. This ensures that a failure in the monitoring system such as a lack of warning cannot be misconstrued as an indication of system readiness. Therefore, it is most effective to provide a positive signal to the vehicle operator so that the driver of a truck, for instance, may be assured of the tire condition before leaving the freight terminal and proceeding onto a highway. The driver of a large truck often cannot determine the condition of his tires by the reaction of the vehicle and may proceed to the point where a soft tire becomes overheated and is completely destroyed. Similarly, on dual wheels, the other tire installed with a malfunctioning tire may be overloaded and destroyed. This economic loss is a significant part of tire expense and may be alleviated by an adequate warning system. If a front tire on a truck is operated at low pressure and a blowout occurs, the consequent loss of control may lead to a very serious accident and loss of life, particularly on a busy high speed highway.
Certain other systems, such as in the Lejeune U.S. Pat. No. 3,638,180, use a manually reset mechanism to indicate an abnormal condition. The system of the Kaida U.S. Pat. No. 3,832,681, uses a mechanism operated by centrifugal force and therefore is dependent on vehicle speed for successful operation. To reduce labor and the possibility of human error, the monitoring system should be completely automatic, and immediately reversible with only the pneumatic pressure as an input. It should operate reliably and consistently from a speed of a few miles per hour up to the top speed at which the vehicle is to operate. Other systems, such as in Yamasaki U.S. Pat. No. 3,828,309, do not account for the centrifugal forces which will be present in a rapidly rotating body since they utilize actuators operating in a radial direction with respect to the axis of rotation. The centrifugal force on a one pound object six inches from the center of the wheel of a passenger car traveling at 60 mph is over 100 pounds. These forces and the friction that they cause must be accounted for in order to provide a reliable indication independent of vehicle speed. Hence, all operating mechanisms should be dynamically balanced and should provide enough operating force to overcome friction inherent in a rapidly rotating body.
Most systems utilizing a magnetic field projected from the rotating wheel to a sensor on the vehicle chassis have not accounted for the influence of nearby ferrous materials on the magnetic field. Almost all vehicles use large quantities of steel and iron for the wheels, tire rims, brake drums, wheel hubs, axles, brakes, springs and other chassis parts. Any of these parts which are near a magnet will tend to distort and divert the magnetic field from its normal condition. Therefore, previous art, such as the magnetic sensors of Ainsworth--U.S. Pat. No. 2,629,086, Cole--U.S. Pat. No. 2,057,556, Lejeune--U.S. Pat. No. 3,638,180, which have not accounted for this type of interference due to the ferromagnetic environment, may have been subject to inaccuracies and variations depending on the specific conditions where they were installed. Other external magnetic influences, although usually quite small, might conceivably influence a magnetic sensor if proper shielding is not provided. In order for a magnetic signal device to be useful and accurate on many different vehicles, means must be provided to shield both the magnetic signal generating unit and the receiving magnetic sensors from the effects of adjacent ferrous components and random magnetic influences.